(1) Field of the Invention
The present invention relates to piezoelectric speakers which radiate acoustic waves by vibrating according to an applied voltage.
(2) Description of the Related Art
Conventionally, speaker arrays, in which many small speakers are arranged, are available. Such speaker arrays control directivity of acoustic waves and realize sound sources that make it possible for users to simultaneously listen to the most appropriate sound (acoustics) at plural listening positions. An electrodynamic method has been mainly used as a method to drive a speaker included in a speaker array.
Recently, in view of the background of miniaturization of audio-visual apparatuses and information equipment, a speaker array has been proposed based on a piezoelectric method to make a speaker array thinner and lighter than a speaker array based on the electrodynamic method. In Non-Patent Reference 1 (High Performance Piezoelectric Mircospeakers and Thin Speaker Array System, ETRI Journal, Vol. 31, No. 6, December 2009), plural piezoelectric microspeakers (piezoelectric speakers) are arranged. With this, the speaker array strikes a balance between being thinner and controlling desired directivity.
Moreover, separately from the above-mentioned background, there is a piezoelectric speaker described in Patent Reference 1 (Japanese Unexamined Patent Application Publication No. 9-327094) as a speaker which combines miniaturization of the piezoelectric speaker itself with planarization of frequency characteristics. In this piezoelectric speaker, plural electrode regions are provided on a diaphragm formed by a piezoelectric material and an area of vibration varies at every frequency band by a circuit network including inductors.
FIG. 15 is a diagram showing the piezoelectric speaker according to Patent Reference 1. A piezoelectric speaker 910 illustrated in FIG. 15 includes a plate-like diaphragm 912 formed by a piezoelectric material.
On the diaphragm 912, an electrode 916 and five electrode portions 914a to 914e are formed. By a circuit network 918, a signal is inputted between the electrode 916 and the five electrode portions 914a to 914e. At this time, via an inductor 922 of the circuit network 918, a signal is inputted to the two electrode portions 914a and 914e. Moreover, via an inductor 920 of the circuit 918, a signal is inputted to the two electrode portions 914b and 914d. A signal is directly inputted to the electrode portion 914c without relying on these inductors 920 and 922.
Usually, as a frequency becomes higher, impedance of a piezoelectric speaker becomes smaller. Meanwhile, in the piezoelectric speaker 910 described in Patent Reference 1, the impedance of inductors 920 and 922 becomes larger as the signal frequency becomes higher. Therefore, as the signal frequency becomes higher, an area of vibration becomes smaller. With this, a variation in sound pressure caused by a variation in frequency can be reduced.